1. Field of the Invention
The invention relates to a touch control apparatus and analog-to-digital converting apparatus and method thereof, more particularly to a touch control apparatus and analog-to-digital converting apparatus and method thereof that are not affected by power fluctuation.
2. Description of the Related Art
Referring to FIG. 1, a touch panel 11, which includes a resistor grid (not shown), is applied with a touch panel power voltage (TPVDD) and a touch panel ground voltage (TPGND) respectively at two first sides opposite to each other in a first direction (X) and respectively at two second sides opposite to each other in a second direction (Y) transverse to the first direction (X). When the touch panel 11 is pressed, based on the principle of voltage division through resistors, divided voltages according to the touch panel power voltage (TPVDD) and the touch panel ground voltage (TPGND) and corresponding to coordinates of the pressed position of the touch panel 11 in the first and second directions (X, Y) will be generated.
Referring to FIG. 2, a handheld mobile communications device that incorporates the touch panel 11 uses a successive approximation analog-to-digital converter 12 to convert a divided voltage corresponding to a coordinate of the touch panel 11 in one direction into a digital value. Another divided voltage corresponding to a coordinate of the touch panel 11 in the other direction is also converted into a digital value in the same manner.
Through a binary search scheme, the successive approximation analog-to-digital converter 12 generates a significant bit, starting from the most significant bit to the least significant bit, for each half clock-pulse cycle. Therefore, if the digital value has N bits, N/2 clock-pulse cycles are required to complete the conversion.
The successive approximation analog-to-digital converter 12 includes a digital-to-analog (D/A) converting unit 121, a comparing unit 122, and a control unit 123. The D/A converting unit 121 converts a digital value from the control unit 123 into a comparison voltage ranging between the touch panel power voltage (TPVDD) and the touch panel ground voltage (TPGND). The comparing unit 122 compares an input voltage (which is a divided voltage generated by the touch panel 11) with the comparison voltage from the D/A converting unit 121. Based on the comparison result determined by the comparing unit 122, the control unit 123 adjusts the digital value that is provided to the D/A converting unit 121.
If the input voltage source changes before the successive approximation analog-to-digital converter 12 is able to complete the conversion, the digital value thus converted will be erroneous. To alleviate this drawback, a sample-and-hold circuit 13 is provided between the touch panel 11 and the successive approximation analog-to-digital converter 12, as best shown in FIG. 3. The sample-and-hold circuit 13 samples a received voltage before conversion by the successive approximation analog-to-digital converter 12, and holds the sampled voltage during conversion by the successive approximation analog-to-digital converter 12.
However, when the handheld mobile communications device transmits and receives signals, there is interference to the touch panel power voltage (TPVDD) such that the touch panel power voltage (TPVDD) fluctuates. The divided voltage generated by the touch panel 11 has a linear response to fluctuation of the touch panel power voltage (TPVDD). When the sample-and-hold circuit 13 is not included (see FIG. 2), the ratio of the input voltage of the successive approximation analog-to-digital converter 12 to the touch panel power voltage (TPVDD) is not affected by fluctuation of the touch panel power voltage (TPVDD), and a correct digital value can be obtained. However, when the sample-and-hold circuit 13 is included (see FIG. 3), the ratio of the input voltage of the successive approximation analog-to-digital converter 12 to the touch panel power voltage (TPVDD) will be affected by fluctuation of the touch panel power voltage (TPVDD), and an incorrect digital value will be obtained.